Dental composites, which essentially comprise a mixture of a polymerizable resin and a glassy filler, have been developing since the early 1970's, when the first materials of this class were introduced. See for example R. L. Bowen et al., "A new series of X-ray-opaque reinforcing fillers for composite materials, J. Dental Research, vol. 51(1) 1972. Until this time, fillings had been based on silver-mercury amalgams, mixtures of acid leachable glass with phosphoric acid (known as "silicate cements"), or unfilled polymerizable resins, and each class of material has certain strengths and weaknesses. For instance, amalgams are generally considered to be cheap and easy to use, and to have a long lifetime due to their strength and high resistance to wear. Disadvantages of amalgam are toxicity of the mercury and the black colour of the filling. Silicate fillings were approximately tooth coloured and released fluoride into the tooth to help prevent a recurrence of decay. However they tended to dissolve quickly and were weak, and are barely used nowadays. Unfilled resins brought advantages of toughness, convenience, and aesthetics, but were still weak, limiting their use to areas of low stress. These unfilled resins also have a high volume shrinkage, commonly at least 5%. This leads to formation of gaps between the filling and the tooth, and subsequent recurring decay of the tooth around and underneath the restoration. The introduction of composite materials brought improvements in surface hardness, higher physical strengths, good aesthetics, lower shrinkage, and also higher resistance to wear. However the wear rate of these composite materials is still higher than of amalgam, and their shrinkage of around 2 to 3 volume percent still leads to gap formation and recurrent caries. It is an aim of many researchers in the dental area to develop composite materials with higher strength, reduced shrinkage and higher resistance to wear, which may be used in place of amalgam. Preferably the material should also be extrudable from a dental syringe since this procedure is not only convenient and time saving for the dentist, but also helps to avoid the inclusion of air bubbles in the cavity.
The present invention provides composite materials with low shrinkage and high surface hardness, and a method for preparing these composite materials. Within certain ranges of the invention materials are provided which may also be extruded from a dental syringe designed for this use.